Centrifuges are used to separate a mixture of substances according to the mass of each component. A centrifuge includes a rotor having a plurality of chambers to receive sample containers. The chambers hold the sample containers at some fixed angle, so that as the rotor is revolved heavier components separate from lighter components.
A rotor is generally shaped like a cylinder or a cone and is removably rested upon a drive hub. A recess is provided at the base of the rotor to receive the drive hub. Originally, the hub-receiving recess of the rotor had a cylindrical configuration to receive a cylindrical hub. Such a configuration is shown in U.S. Pat. No. 4,205,261 to Franklin, assigned to the assignee of the present invention. However, modern centrifuge systems provide an ever increasing angular velocities. The modern high-speed centrifuge systems, referred to as ultracentrifuges, render cylindrical fittings less satisfactory than in lower speed centrifuges. Thus, high-speed centrifuges include a frustroconically-shaped fitting arrangement in place of a cylindrically-shaped fitting arrangement. The frustroconical shape creates a self-centering effect for the rotor seated on the drive hub. For example, the hub-engaging surface of a rotor may be at a fourteen degree angle, more or less, to the vertical. Then in manufacturing the drive hub, the upper extent of the rotor-engaging surface is curved sharply to center and support the rotor. Consequently, a line of contact is created between the drive hub and the rotor.
An ultracentrifuge rotor may experience 600,000 g or higher forces which cause the rotor to expand in size. Under full rotational speed the hub-engaging surface of the rotor increases in diameter to a much larger extent than the hub, causing the rotor to drop slightly on the hub. When the centrifuge is stopped, the rotor returns to an unstrained condition and the hub-engaging surface reduces in diameter giving rise to an interference fit with the hub along the above-described line of contact. It may take a considerable axial force to free the rotor from the hub. This is a very undesirable condition since a centrifuge separation is easily disturbed by jarring and rough handling. Furthermore, damage to the centrifuge may result from shock on the drive bearings and possible bending of the rather delicate spindle. Moreover, as the rotor expands, a small amount of tilting of the rotor on the drive hub is possible. When tilted even slightly, the contact of the hub against the rotor becomes a diametrically opposing two-point contact, rather than the intended line of contact. Thus, when the rotor drops down on the hub, the local stresses at the two contacting points will be very high and there will be a greater tendency for the hub to become embedded in the rotor.
Tapering the outer periphery of the skirt to match the taper of the rotor somewhat reduces the problem of rotor sticking. Corresponding tapers, however, place an emphasis on rigid manufacturing tolerances. Otherwise, there are still a number of possible conditions which will result in high local stresses at the hub-rotor interface which, in turn, tend to cause sticking. It is possible, for example, to have the actual mating contact at either the large diameter end or the small diameter end. Thus, the problem associated with tilting is not eliminated by employing a hub with a matching taper, since a two-point contact may still result from this configuration.
Therefore, it is an object of the present invention to design a centrifuge hub for drive engagement with rotors, wherein the force required to free a rotor from the hub is reduced. It is further an object of the present invention to eliminate the possibility of a two-point contact between a hub and a tilted rotor.